1. Field of the Invention
The present invention relates to a multi-beam scanning optical system. In particular, the present invention relates to a multi-beam scanning optical system suitable to an image forming apparatus such as a laser beam printer, a digital copying machine, or a multifunction printer, which employs, for example, an electrophotographic process, in which a plurality of light fluxes (laser light) emitted from a light source unit are deflected and reflected on a polygon mirror serving as an optical deflector and then a surface to be scanned is scanned with the light flux through a scanning optical unit to record image information.
2. Related Background Art
Up to now, in a scanning optical system used for an image forming apparatus, a light flux emitted from a light source (light-emitting region) serving as a light source unit is guided to a deflection unit through an incident optical unit including a collimator lens and a cylindrical lens (cylinder lens) . The light flux which is deflected and reflected on the deflection unit is imaged in a spot shape onto a surface to be scanned through a scanning optical unit, and optical scanning using the light flux is performed.
In recent years, with improving the performance and the function of the image forming apparatus, a need for high speed scanning of the scanning optical system has been increased. For meeting such a need for high speed scanning, one solution is to use a plurality of light sources. For example, a scanning optical system has been proposed, in which a multi-beam laser chip, which emits a plurality of light fluxes aligned on a straight line from a single chip, is used as a light source (see, for example, Japanese Patent Application No. H09-054263).
Now, in the case where the surface to be scanned is flat or cylindrical, when there is an instant at which a light flux is perpendicularly entered into the surface to be scanned, return light to a light source is generated. In the case where such a phenomenon is caused, a streak or the like is caused on a printed image in some cases. To prevent the phenomenon, as shown in FIGS. 11A and 11B, the light flux is generally allowed to enter at an angle relative to a sub-scanning direction. Note that, in FIGS. 11A and 11B, reference numeral 98 denotes a housing member that houses the light source, an optical deflector, and the like and reference numeral 99 denotes a photosensitive belt or a photosensitive drum, which serves as an image bearing member.
However, when a plurality of scanning lines are drawn at the same time in the cases shown in FIGS. 11A and 11B, because an imaging position is deviated in the sub-scanning direction, the optical path lengths of respective light fluxes are different from one another as shown in FIGS. 12A and 12B. As a result, the lengths of the scanning lines are not equal to one another.
FIG. 12A is a schematic view showing a state in which four light fluxes are entered into the surface of the cylindrical photosensitive drum 99. In the case shown in FIG. 12A, an optical path length of a light flux located on the uppermost side in the figure becomes longer than an optical path length of a light flux located on the lowermost side by a distance D. At this time, with respect to a region other than a central region of the photosensitive drum 99, the respective light fluxes are entered into the surface of the photosensitive drum 99 at an angle relative to a main-scanning direction. Therefore, the incident positions of the respective light fluxes are deviated in the main-scanning direction according to a difference among the optical path lengths.
FIG. 12B shows a relationship among the lengths of the scanning lines in such a state. A scanning line located on the right side in FIG. 12B indicates a scanning line drawn by the light flux located on the uppermost side in FIG. 12A. In addition, a scanning line located on the left side in FIG. 12B indicates a scanning line drawn by the light flux located on the lowermost side in FIG. 12A. As shown in FIG. 12B, a difference between a length of the scanning line drawn by the light flux having the longest optical path length and a length of the scanning line drawn by the light flux having the shortest optical path length corresponds to a distance J on each side. In the case where such a phenomenon is neglected, a printing quality deteriorates on both sides of a printed image.